Laboratory Techniques: using a micropipette and preparing solutions, serial dilution and concentration calculations, culturing and plating bacteria, and running a bioreactor for large-scale fermentation

What this module is about

Biotechnology is a practical subject, and this module covers the bench skills that the school-based practical assessment rewards. You learn to measure tiny volumes accurately, to dilute and quantify, to grow bacteria so they form countable colonies, and to scale a culture up in a bioreactor. These skills turn the ideas from earlier modules into things you can actually do, and the maths here (dilutions and concentrations) is a reliable source of marks.

The full set of dot points for this module is at /sg-o-level/biotechnology/syllabus/laboratory-techniques.

Measuring and mixing: micropipettes and solutions

The dot point on micropipetting and solution preparation covers the most-used tool on the bench. You set the volume within range, fit a fresh tip, press to the first stop to draw up, and press to the second stop to dispense fully, using a fresh tip each time to avoid cross-contamination. Solution preparation is the partner skill: making up a solution to a required concentration by dissolving the right mass of solute in the right volume.

The key relationship for concentration is:

so to make a target concentration you rearrange to find the mass or volume you need.

Diluting and counting: serial dilution

The dot point on serial dilution and concentration is the module's main calculation skill. A serial dilution repeats the same dilution step, for example transferring 1 part of culture into 9 parts of diluent for a ten-fold dilution at each step. The total dilution factor multiplies across the steps, so the final concentration is the original divided by the total dilution factor. The same idea lets you count cells: dilute a dense culture until colonies are countable, then multiply back by the dilution factor to find the original cell number.

Growing bacteria: culture and plating

The dot point on bacterial culture and plating covers growing bacteria on agar. Streaking drags a loop across the plate to progressively thin the cells so single, well-separated colonies grow, ideal for isolating a pure colony. Spreading distributes a measured volume of diluted culture evenly so the colonies can be counted, which connects directly to the dilution calculation above. Both rely on the aseptic technique from the Cells and Microorganisms module.

Scaling up: bioreactors and fermentation

The dot point on bioreactors and fermentation takes the culture from a plate to industrial scale. A bioreactor (fermenter) is a large vessel that grows microorganisms or cells under controlled conditions. It controls temperature, pH, oxygen supply through aeration and stirring, and nutrient levels, and is kept sterile, so the culture grows efficiently and yields the maximum product. This is where the log-phase growth and aseptic ideas from earlier modules pay off commercially.

How the module fits together

• Accuracy first. A correctly set micropipette with a fresh tip underpins every other technique.
• Dilute to quantify. Serial dilution makes dense cultures countable; multiply back by the dilution factor.
• Plate to grow. Streaking isolates; spreading enables counting.
• Scale to produce. A bioreactor controls temperature, pH, oxygen and nutrients to maximise yield.

Check your knowledge

A mix of recall and calculation across the four dot points. Try them timed, then check the solutions.

1. State two ways to keep micropipetting accurate and free of contamination. (2 marks)
2. A culture is diluted by four ten-fold steps. State the total dilution factor. (1 mark)
3. You spread 0.1 mL of a final dilution (total dilution factor 1000) and count 30 colonies. Calculate the cells per mL in the original. (2 marks)
4. State the difference between streaking and spreading bacteria on agar. (2 marks)
5. Name three conditions a bioreactor controls. (2 marks)